The present invention relates to adaptive closed-loop control systems and, in particular, to phase-lock loops.
Phase-lock loops are used in modems and other communications devices in order to perform coherent demodulation. Specifically, one needs to be able to accurately track the frequency and phase of the carrier of an incoming signal in order to accurately recover the information contained in that signal.
One of the issues confronting the designer of phase-lock loops in such applications relates to the bandwidth of the loop. Ideally, one would like to minimize the bandwidth of the loop so as to be able to accurately track the signal in question while minimizing noise in the loop. On the other hand, the bandwidth must be sufficient to ensure that the full range of expected signals can be tracked. The resulting required design trade-off means that the performance of the loop may be sub-optimal.
Another issue relates to the fact that the loop may be confronted with signals whose parameters span a wide range, and the designer must take account of this when designing the circuit. For example, on the one hand, one would like to represent the signals being operated on with some minimum level of precision. On the other hand, achieving this level of precision across a wide range of signals can result in inordinately high circuit costs due to the need to use very large registers (for digital implementations) or very-high-precision amplifiers and other components (for analog implementations). As a result, trade-offs are often required which may, in turn, again result in sub-optimal performance.
One specific application in which these issues arise is in the tracking of frequency offset in the echoes which are returned to echo-canceling modems. The problem comes about because the echoes may experience frequency translation while traversing the telephone network. Thus, when they return to their source, there may be a discrepancy between the echo carrier frequency and the local carrier frequency. Unless such discrepancy is accounted for in the echo cancellation process, unacceptable performance may result. Accordingly, a phase-lock loop is typically used to generate an estimate of this frequency discrepancy--usually referred to as the "frequency offset" or "phase roll."